Small Craft Advisory 480, NWS Juneau, AK

SOUTHEAST ALASKA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE JUNEAU AK
416 PM AKDT THU AUG 30 2012

.SHORT TERM...AN UPPER LEVEL RIDGE ORIENTED FROM SW TO NE ACROSS
THE NORTH-CENTRAL PANHANDLE WILL SLOWLY SHIFT SE THROUGH THE
PERIOD. AT THE SURFACE A RIDGE CURRENTLY OVER THE EASTERN GULF
WILL CONTINUE TO BUILD ALONG THE PANHANDLE THIS EVENING AND WILL
REMAIN THROUGH THE DAY FRIDAY. THIS RIDGE WILL KEEP A GENERALLY
SOUTHERLY FLOW THROUGH THE INSIDE WATERS AND CAUSE 20-25KT WINDS
IN NORTHERN LYNN CANAL THIS AFTN/EVE AND AGAIN FRIDAY AFTN WITH
ADDED SEA BREEZE AFFECTS. THE HIGH PRESSURE ALOFT WILL CONTINUE
THE MOSTLY CLEAR SKIES OVER LAND BUT MAY CAUSE A MARINE LAYER TO
DEVELOP TONIGHT. MAXIMUM RADIATIONAL COOLING OVERNIGHT...AS WITH
LAST NIGHT WILL ALLOW TEMPERATURES TO DROP INTO THE UPPER 30S TO
LOW 40S. COLDEST AREAS WILL BE IN WIND SHELTERED VALLEY LOCATIONS
WITH MARINE LOCATIONS BEING SEVERAL DEGREES WARMER. HAVE KEPT
MENTION OF FOG IN THE FORECAST FOR THE LATE NIGHT/EARLY MORNING
TIME FRAME. REMOVED IT HOWEVER FROM AREAS THAT ARE EXPECTED TO
GET SOME WIND AND ADDED IT TO A FEW OF THE MARINE AREAS INCLUDING
CLARENCE AND SUMNER STRAITS.

AN UPPER LEVEL TROUGH OF LOW PRESSURE WILL MOVE INTO THE WESTERN
GULF OVERNIGHT ALONG WITH ITS ASSOC SURFACE FRONTAL SYSTEM. THIS
FRONT WILL MOVE INTO THE YAKUTAT AREA WITH RAIN LIKELY BY FRIDAY
AFTERNOON. WINDS ALONG THE FRONT ARE EXPECTED TO INCREASE 20-25KT
OVER THE OUTSIDE WATERS WITH AREA 52 SEEING WINDS OF UP TO 30KT.
HIGH CLOUDS AHEAD OF THIS FRONT WILL REACH A HAINES/GUSTAVUS/SITKA
LINE BY THE AFTERNOON AS WELL.

BIGGEST FORECAST CHALLENGE TODAY WAS WITH TEMPERATURES AND FOG.
USED LAST NIGHTS TEMPERATURES TO GUIDE HOW COLD TO GO
TONIGHT...BUT DID NOT GO AS COLD FOR YAKUTAT AREA AS CLOUDS WILL
BE MOVING IN THERE AND LIMIT THE AMOUNT OF RADIATIONAL COOLING.
INCREASED TEMPERATURES FOR FRIDAY SLIGHTLY FROM PREVIOUS FORECAST
DUE TO ANOTHER MOSTLY SUNNY DAY EXPECTED. NUMERICAL MODELS WERE IN
FAIRLY GOOD AGREEMENT IN THE SHORT TERM. USED A BLEND OF THE
NAM12/GFS/ECMWF WITH LOCAL ADJUSTMENTS THEREAFTER. FORECASTER
CONFIDENCE IS ABOVE AVERAGE.

&&

.LONG TERM...

SURFACE RIDGE EXTENDING NORTHWEST ACROSS SOUTHEAST ALASKA BEGINS TO
BREAK DOWN FRIDAY NIGHT AS WEATHER FRONT MOVES IN FROM THE GULF OF
ALASKA. THE FRONT WILL SLOW DOWN AS IT APPROACHES THE COAST FRIDAY
NIGHT AND THUS WILL SUBJECT MOST OF THE PRECIPITATION TO YAKUTAT
EARLY. RAIN WILL DELAY MOVING INTO THE PANHANDLE THROUGH SATURDAY
AFTERNOON AND TO SKAGWAY AND THE FAR SOUTHEASTERN CORNERS UNTIL
SATURDAY NIGHT. OFFSHORE, WINDS ASSOCIATED WITH THE BARRIER JET
WILL INCREASE TO GALE FORCE OVER MARINE ZONE 52 AND TO SMALL CRAFT
LEVELS OVER MOST OF THE OFFSHORE MARINE ZONES FRIDAY NIGHT AND
SATURDAY FROM WEST TO EAST. ONLY CONCERNED ABOUT FOG IN THE FIRST
PERIOD OF THE EXTENDED AS MARINE STRATUS SHOULD BE WASHED AWAY
WITH ONSET OF RAIN AND INCREASING WINDS FROM THE SOUTHEAST.

POST-FRONTAL PASSAGE, FLOW BECOMES MORE SOUTHWESTERLY ON SUNDAY SO
WENT WITH CONVECTIVE WORDING USED RATHER THAN STRATIFORM. RAIN
CHANCES STILL IN THE LIKELY CATEGORY FOR MOST OF THE CENTRAL AND
NORTHERN ZONES. WEAK RIDGING MAY KEEP THE SOUTHERN ZONES A LITTLE
MORE DRY. WIND WISE IN THE EXTENDED MOST OF THE INSIDE PASSAGES
WERE KEPT CONSTANTLY SOUTHERLY THROUGH THE END OF THE MARINE
FORECAST PERIOD. IN THE FAR EXTENDED GFS DIVERGED BETWEEN
RUNS...WITH 06Z DEVELOPING A STORM IN THE SOUTHEASTERN BERING
WHILE THE 12Z ELIMINATING IT EARLY NEXT WEEK. THIS MAY AFFECT
GREATLY OUR PRECIPITATION BY THURSDAY. THUS CONFIDENCE BEYOND
TUESDAY NOT SO GREAT.

WENT WITH NAM BLENDED WITH EC, AND GFS THROUGH SUNDAY NIGHT FOR POP
AND PRESSURE FIELDS. FROM MONDAY ON...WENT WITH HPC GUIDANCE MAINLY
WITH A LITTLE BIT OF GFS AND EC ADDED IN AT VARIOUS TIMES.

.AJK WATCHES/WARNINGS/ADVISORIES...
PUBLIC...NONE.
MARINE...SMALL CRAFT ADVISORY FOR PKZ012-051-052.

&&

HANNON/EAL/JWA